The invention relates to the refractory system for use in a cyclone or other cylindrical, conical or truncated conical shaped apparatus that handles very hot gases with entrained solids. The cyclone or cyclone separator refers to to a funnel shape device for removing particles from air or other fluids by centrifugal means. Such apparatus is used to remove dust from air or other fluids, steam from water, and water from steam, and certain applications to separate particles into two or more size classes. The cyclone is used to collect the solids entrained in the fluid stream. The present invention has particular application for use in collecting the solids entrained in the hot gases having a furnace of a circulating fluidized bed steam generator, it will be understood that the invention also has application to other cylindrical, conical or truncated conical chambers that handle hot materials and which must be lined with refractory materials because of the high temperatures within the chamber.
Fluidized bed apparatus is being increasingly utilized for a wide variety of applications. The use of a circulating fluidized bed is particularly advantageous because of technological developments which have resulted in significant advances in both operating and fuel flexibility. While the present invention has primary application to a combustion process in a steam generating system, it will be understood that the present invention may also be used in a wide variety of fluidized bed apparatus. Fluidized bed combustion apparatus can bum coal efficiently at temperatures low enough to avoid many of the problems of combustion in other modes. The term "fluidized bed" refers to the condition in which solid materials are given free flowing, fluid-like behavior. As a gas is passed upward through a bed of solid particles, the flow of gas produces forces which tend to separate the particles from one another. At low gas flows, the particles remain in contact with other solids and tend to resist movement. This condition is referred to as a fixed bed. As the gas flow is increased, a point is reached at which the forces on the particles are just sufficient to cause separation. The bed is then deemed to be fluidized. The gas cushion between the solids allows the particles to move freely, giving the bed a liquid-like characteristic.
The fluidizing gas is generally combustion air and the gaseous products of combustion. Two main types of fluidized bed combustion systems are (1) bubbling fluid bed (BFB) in which the air in excess of that required to fluidize the bed passes through the bed in the form of bubbles The bubbling fluid bed is further characterized by modest bed solids mixing rate and relatively low solids entrainment in the flue gas and (2) circulating fluid bed (CFB) which is characterized by higher velocities and finer bed particle sizes. In such systems the fluid bed surface becomes diffused as solids entrainment increases, such that there is no longer a defined bed height. Circulating fluid bed systems have a high rate of material circulating from the furnace to the particle recycle system and back to the furnace. Characteristics of apparatus of this general type are further described in the publication Combustion Fossil Power, edited by Joseph G. Singer, P.E. and published by Combustion Engineering, Inc.; a subsidiary of Asea Brown Boveri, 1000 Prospect Hill Road, Windsor, Conn. 06095, 1991.
In the refractory brick and masonry field it is well-known that a brick lining system will be self supporting when properly installed in a cylindrical configuration contained within a rigid cylindrical steel or high strength concrete structural hoop shaped enclosure. Such refractory brick installations typically include truncated pie shaped bricks having the widest part thereof disposed farthest from the center of the enclosure. In a typical refractory brick array, such as the recycle cyclone of a circulating fluid bed boiler, the structural stability of the lining system is further enhanced by the thermal expansion of the refractory lining with increasing temperature. As the temperature increases the individual bricks expand and even more firmly lock the bricks in place. In a properly designed system at full operating temperature, each individual brick will be in compression and the skin of the external enclosure will be in tension. The stresses are collectively referred to as "hoop stresses". The stability of such a system is dependent on the balance between the two opposing sets of compressive and tensile stresses. When a cylindrical brick lined enclosure is penetrated by any kind of opening it is essential to provide a method to redirect or transfer the hoop stress around the discontinuity inherent in the opening.
In the recycle cyclone of a typical fluid bed boiler, a rectangular cross section inlet duct connects the furnace to the recycle cyclone. The inlet duct intersects the cylindrical (barrel) section near the top of the cyclone. The outside (the side farthest from the geometric axis of the cyclone) wall of the inlet duct is tangent to the steel cylinder which forms the wall of the cyclone barrel. The inside wall of the inlet duct forms an acute angle to the barrel section. Since the cylindrical part of the cyclone is broken by the flat vertical walls of the inlet duct, the inherent hoop stress of the hot face lining must be accommodated in a manner that will insure stability of the system.
In older recycle cyclone refractory lining systems the stresses associated with such discontinuities were transferred to the steel skin of the cyclone enclosure by means of a metallic assembly formed by two steel flange faces fixed at a 90 agree angle. The steel flange faces, in such structures, are braced by a gusset plate web. Such systems have two inherent limitations. First, because of the mass of the steel flange and web, a significant amount of heat is transferred to the outside steel enclosure. This is undesirable because it compromises the efficiency of the thermodynamic system. Second, because of the physical limitations of steel and alloy building materials, the metallic stop block cannot be exposed to the full heat of the gases and solids flowing into the cyclone. The properties of metals, such as steel, that limit use of such metals in such temperature environments include (a) the allowable design stress of a steel component decreases as the temperature increases: (b) long term exposure to high temperature causes detrimental changes in the grain structure of steels and alloys; (c) the rate of heat transfer in a metallic material is a least 10 to 20 times higher than that of a high strength refractory material; and (d) the coefficient of thermal expansion of steel is 3 to 4 times greater than that of typical refractory material. Since the thermal stresses are greatest at the inside surface of the refractory and since the steel support used in the prior art must be positioned back from that face, no support can be placed at the most mechanically advantageous position to resist the thermal stress loading of the inner (and hottest) refractory face.
The design of structure that can physically be placed at the most mechanically advantageous position and which can absorb the hoop stresses is complicated by the non-planar, moment force at the hot face of the lining systems as the result of the varying tensile and compressive forces within the hoop structure.
Another problem with the prior art structure is that the that structure does not equally accommodate tensile and compressive forces because of the geometric shaped employed.